Extend present serological typing systems to identify new cell surface antigen systems specified by various classes of endogenous MuLV, using techniques developed in the AKR-Gross MuLV system. Characterize new antigens biochemically and determine relationship to viral structural components. Conduct detailed immunogenetic and virological analyses of preleukemic phase of AKR mice. Use genetic approach to analyze age-dependent amplification of MuLV antigenic expression in thymus of six month old AKR mice. Determine biological and biochemical character and significance of MuLV isolated from AKR mice as listed above. Use cell fractionation techniques to identify the cellular source of various classes of MuLV from thymus of different age AKR mice.